A simple sensor for kHz-rate simultaneous measurement of OH and temperature in combustion based on UV broadband absorption spectroscopy

Abstract

Temperature and hydroxyl (OH) radical concentration are critical parameters in combustion processes. However, current measurement techniques often rely on ultraviolet (UV) lasers, which involve complex optical setups and high costs, thereby limiting their applicability in harsh industrial conditions. This study introduces a simple and cost-effective sensor based on UV broadband absorption spectroscopy (BAS) using a 308 nm light-emitting diode (LED) and a portable spectrometer, capable of performing kHz-rate dynamic measurements of temperature and OH concentration. To reduce the processing time of BAS multi-line spectra, two methods are presented: a polynomial method (accuracy 10⁻⁷, 35 ms computation time) and a neural network method (accuracy 10⁻³, $15\mu s$ computation time). The developed BAS sensor was evaluated in methane/air premixed flat flames with varying equivalence ratios (0.8 to 1.2) and in a thermal wind tunnel with different total temperatures (1373 K to 1873 K). Time-averaged results in flat flames at different heights showed good agreement with computational fluid dynamics (CFD) simulations. Under stable fuel-lean conditions, kHz-rate measurements exhibited high precision, with repeatability errors (1$\sigma$) of 29 K (1.7%) and 49 ppm (2.5%). In unstable fuel-rich conditions, fast Fourier transform (FFT) analysis revealed distinct frequency characteristics. In the thermal wind tunnel, time-averaged temperature results closely matched thermocouple data in the stabilization section. The precision of kHz-rate temperature measurements remained comparable to that of laboratory flat flame experiments, with a temperature repeatability error (1$\sigma$) of 52 K (2.8%) at 1900 K. FFT analysis also clearly demonstrated the frequency characteristics. Experiments conducted in both laboratory and thermal wind tunnel environments validated the sensor’s accuracy and high-rate dynamic measurement capability, affirming its potential for diverse applications and providing a new technical reference for industrial measurement of temperature and OH concentration.

Novelty and significance

This study proposes a novel temperature and OH concentration sensor, which offers the advantages of simplicity, low cost, and robust environmental adaptability. Additionally, two fast data processing methods with ms-level and $\mu$s-level computational times were developed. High-precision (2.8%) simultaneous kHz-rate dynamic measurements of temperature and OH concentration were achieved in the harsh industrial environment of the thermal wind tunnel using the proposed sensor. In summary, this study provides a new approach for dynamic measurement of temperature and OH concentration in harsh environments.